It's not really clear. Are you born with two natures inherently, or can you learn another. I think you have to be born with an affinity and then you learn you have the other... idk.
naruto technically can but i don't think we'll see it just yet.
however he would learn it much faster then anyother shinobi because he trains with shadow clones which in the data book says what ever they gain or learn is passed to the user upon death or release
My guess will be that he will learn water next. A counter balance to Sasuke's fire techs. Plus recall hearing that the word "Shippuden" means hurricane, or something.